1. Field of the Invention
The present invention relates to a semiconductor film having a heterostructure, a semiconductor element in which the semiconductor film having a heterostructure is used, and a semiconductor device in which the semiconductor film having a heterostructure is used.
Note that in this specification, a semiconductor film refers to a semiconductor in a film shape, a semiconductor element refers to general elements which function by utilizing semiconductor characteristics, and a semiconductor device refers to general devices using the semiconductor element. Thus, electro-optical devices, semiconductor circuits, and electronic devices are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, techniques in which thin film transistors (TFTs) are formed using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) which is formed over a substrate having an insulating surface have attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed. In addition, there are various kinds of metal oxides, which are used for a wide range of applications. Indium oxide is, for example, a well-known material and is used as a light-transmitting electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Examples of a metal oxide having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Such a metal oxide having semiconductor characteristics can be used for a variety of semiconductor elements such as a sensor and a transistor as well as a transparent conductive film. A thin film transistor in which a channel formation region is formed using such a metal oxide in a film shape is already known (Patent Documents 1 and 2).